Photodetectors typically have a photoconductive semiconductor material, for examples, silicon (Si) and gallium arsenide (GaAs). Considerations in choosing a semiconductor material for a particular application include its energy gap, which in turn determines the range of wavelengths that can be detected, the time response, and the optical sensitivity of the material. The performance of a photodetector may be judged by various criteria including sensitivity. Sensitivity refers to the current produced by a photodetector with respect to the electromagnetic power. A photodetector with high sensitivity will produce more current for a given intensity of incident radiation than one with a low sensitivity. Sensitivity is affected by several factors including the mobility of the electrons in the material. Semiconductor materials with a higher mobility have a higher sensitivity because the charge carriers can move at a greater speed.
One type of conventional photodetector, illustrated in FIG. 1A, includes a semiconductor material with a pair of contact electrodes on either side of the semiconductor material. The semiconductor material, upon which radiation is incident through the top contact electrode, acts as a direct conversion layer to convert incident radiation to electric currents. A voltage source connected to the electrodes applies a positive bias voltage across the semiconductor material, and current is observed as an indication of the magnitude of incident radiation. When no radiation is present, the resistance of the semiconductor material is high for most photoconductors, and only a small dark current can be measured. When radiation is made incident through the top contact electrode upon the semiconductor material, electron-hole pairs form and drift apart under the influence of a voltage across that region. Electrons are drawn toward the more positively (+) biased contact electrode and holes are drawn toward the more negatively biased (quasi-ground) contact electrode. Formation of electron-hole pairs occurs due to interaction between the incident radiation and the semiconductor material. If the x-rays have energy greater than the bandgap energy of the semiconductor material, then electron-hole pairs are generated in the semiconductor as each photon is absorbed in the material. If a voltage is being continuously applied across the semiconductor material, the electron and hole will tend to separate, thereby creating a current flowing through the photodetector. The magnitude of the current produced in the photodetector is related to the magnitude of the incident radiation received.
After removal of the incident radiation, the charge carriers (electrons and holes) remain for a finite period of time until they either reach the electrodes or recombine. The term “charge carriers” is often used to refer to either the electrons, or holes, or both. The rate at which electrons and holes recombine is called the recombination rate, and is a property of the semiconductor material. The recombination rate limits the response time of the photoconductor. The un-recombined carriers can cause a lingering current due to the excess carriers that remain for a time, even after radiation is removed.
The tradeoff between response time and sensitivity is found in the properties of the semiconductor material itself. The unbound electrons in any semiconductor material have a mean lifetime before they are recombined with a hole. The value of the mean lifetime depends upon the characteristics of the semiconductor material. The faster the rate of recombination, the shorter the response time. Furthermore, the unbound electrons have a mobility figure dependent upon the semiconductor material. Higher mobility materials generally have a greater sensitivity. The resulting tradeoff between response time and sensitivity appears to be a direct result of competing properties (recombination rate vs. electron mobility) of the semiconductor material.
Another type of conventional photodetector is the photodiode, as illustrated in FIG. 1B. A photodiode is composed of a p-doped semiconductor (p-type) material layer and an n-doped semiconductor (n-type) material layer. Light is made incident on the depletion region between the p-type and the n-type material layers, creating electron-hole pairs and thus a current. To control the thickness of the depletion region, a layer of intrinsic (i) material may be inserted between the layer of p-doped semiconductor material and the layer of n-doped semiconductor material. Such a photodiode 100 is termed a “p-i-n” diode for the configuration of semiconductor material in the diode.
In operation of a p-i-n photodiode, a reverse-bias voltage is applied across the photodiode 100 and x-rays are mostly absorbed in the intrinsic region. The electron-hole pairs then separate under the applied electric field and quickly migrate toward their respective poles. The electrons move toward the positive pole and the holes move toward the negative pole. Due to the low recombination rate of the intrinsic region and also due to the high mobility of the intrinsic material, there is little chance that the carriers will recombine before they arrive at the interface with the doped material. The electrons and holes then collect near the respective interface with the doped material. As a result of charge collection, the response of the p-i-n photodiode is capacitivly limited.
One problem with the conventional photodetectors is that they often suffer from poor sensitivity. Photodiode 100 of FIG. 1B may be operated in the avalanche mode of operation. If a large reverse-bias is placed across a photodiode, the free carriers are accelerated to such a high energy that many other electron-hole pairs are created by collision, thus producing a large current for a small amount of incident radiation. Although an avalanche photodiode has increased sensitivity, accurate measurement of the intensity of incident radiation is difficult or impossible, and the response time is only in the nanosecond range. Another problem with conventional photodetectors is that they may have poor radiation hardness.